Through the many years of manned exploration of outer space, the composition and properties of water and other on-board liquids have typically been subject to a monitoring protocol that includes only periodic monitoring for some tests, and lack of monitoring at all for some components. Of course, water and other liquid management systems on board spacecraft tend to be robust, so it has typically been practiced that, absent outward signs of malfunction, testing for contaminants or properties that are out of specification has been thought to be necessary only periodically. This has meant that for short duration flights, no on-board monitoring is typically used. For long duration flights in Earth orbit, the periodic monitoring can be accomplished with samples being collected on-board and sent back to Earth for testing.
Notwithstanding the robustness of on-board water or other liquid management systems, there can be components in water or other liquids that can have a profound effect on personnel or on-board systems so that more frequent on-board monitoring is needed. This was demonstrated recently when silica buildup on spacesuit water separators caused a buildup of water inside the suit, necessitating an emergency abort of an astronaut spacewalk on the International Space Station. Additionally, the prospect of manned exploration of deep space will preclude the use of terrestrial testing, again necessitating that testing be conducted on-board.